The present invention relates to apparatus and methods for remediating materials contaminated with hydrocarbons, and particularly relates to apparatus and methods using a tandem dryer arrangement for sequentially volatilizing the low boiling fraction and the remaining higher boiling fractions of the volatile organic contaminants in the materials undergoing remediation and subsequently eliminating, e.g., oxidizing all such volatilized components. Particularly, the present invention relates to apparatus and methods for remediating soil contaminated with hydrocarbons.
While the preferred embodiment of the present invention is described herein in relation to remediation of soil contaminated with hydrocarbons, it will be appreciated that materials other than soil may be remediated by the practice of this invention. For example, the present invention may be used to eliminate volatile organics from metal turnings, sludges, drilling muds, inorganic chemicals, etc.
Soils are frequently contaminated with volatile organics, i.e., hydrocarbon products, and this constitutes a highly significant and major pollution problem. The contaminants may range from gasoline through heavy hydrocarbon products and hydrocarbon chemicals, such as PCBs. Various efforts have been directed to remediating the soil and one of the most effective is to thermally treat the soil. High cost, however, is an inhibiting factor and, in many cases, is the result of inefficiently designed equipment and limited equipment capacities. For example, a major factor affecting the cost is fuel efficiency as well as the downstream treatment of the residual gaseous components driven off from the soil. Thermal efficiency is often disregarded in many systems. For example, heat recovery and high temperature fume incineration is frequently ignored.
In these prior systems, the basic process for cleaning the soil is to expose it to high temperatures, where the contaminants are volatilized and subsequently oxidized or processed in a reducing environment to leave a carbon char material in the soil. The temperatures at which the soils must be processed can vary substantially from as low as 150.degree. F. to over 1100.degree. F. in order to obtain satisfactory low levels of total residual petroleum hydrocarbons. With these wide-ranging temperatures necessary to clean up a wide variety of contaminants, it is essential to design a remediation system which not only effectively removes the contaminants but does so in a thermally efficient and, hence, fuel-efficient manner.
A significant additional problem in soil remediation resides in the system's ability to separate the soil dust from volatilized hydrocarbons without adversely affecting the efficiency of the separator. A separator for use in soil remediation processes may comprise a baghouse having a series of bags depending from a support structure, the volatilized hydrocarbon gases passing through the bags for subsequent elimination, e.g., oxidation and the dust being separated out for further remediation, e.g., by return to the heated hydrocarbon contaminant-free soil. It will be appreciated that the hydrocarbon contaminants contain low, intermediate and high boiling fractions having different condensation temperatures. Considering the materials of the baghouse filter bags, and without utilizing exotic and, hence, expensive bag materials, the maximum operating temperature of a baghouse is about 400.degree.-450.degree. F. With exotic bag materials, the maximum baghouse temperature may reach about 550.degree. F. Such exotic materials, however, have decreased separation capacity and are generally not desirable. At or below that maximum operating temperature for the baghouse, it will be appreciated that certain of the intermediate and high boiling fractions will condense. Should this occur, these heavier hydrocarbon fractions will condense either back onto the soil or will be carried into the baghouse and form liquid droplets and eventually an oil film covering the interstices of the bags. Consequently, the efficiency of the baghouse filters deteriorates and the possibility of a fire in the baghouse is substantially increased.
According to the present invention, there is provided a first-stage heating unit, e.g., a rotatable drum dryer. This first dryer preferably has a burner at one end and an inlet at its opposite end for receiving the contaminated soil whereby the contaminated soil is heated, preferably in counterflow relation to the hot gases of combustion. While a rotatable drum dryer is preferred, particularly for soil remediation, other types of dryers may be used such as a fluid bed, flash or a rotary hearth dryer. The first unit is operated in a relatively low temperature range, for example, on the order of 150.degree.-450.degree. F., such that only the low boilers of the low boiling fraction are volatilized. The first unit also evaporates the moisture content of the contaminated soil. The partially remediated soil is then passed to a second-stage heating unit, e.g., a second rotatable drum, which similarly and preferably has a burner at one end and an inlet at the opposite end for receiving the partially contaminated soil from the first-stage heating unit. The soil passes through the second drum, preferably in counterflow relation to the combustion gases. Importantly, the second-stage heating unit is operated at a temperature substantially higher than the operating temperature of the first-stage heating unit. Consequently, the remaining intermediate and high boiling fractions consisting of the intermediate and high boilers are volatilized. The soil exiting the second unit is therefore substantially free of hydrocarbon contaminants and exits the second unit at an elevated temperature. The contaminated free soil is then passed through a cooler, where its temperature is reduced and water is added to return the soil to its proper moisture levels.
A significant feature of the present invention resides in passing the remaining volatilized contaminants from the second-stage heating unit to the burner of the first-stage heating unit. Substantially all of the volatilized contaminants received from the second unit are at least partially oxidized by the burner of the first unit. Thus, the intermediate and high boilers of the second unit are used as a source of fuel for the burner of the first unit. By at least partially oxidizing these contaminants received from the second unit, portions of the intermediate and higher fractions are converted to the lower fractions or lighter oils. Exhaust gases from the first unit thus include the portion of the contaminants volatilized in the first unit, any remaining non-oxidized volatilized portion of the remaining contaminants from the second unit and dust from the contaminated soil. Importantly, the temperature of the exhaust gas stream from the first unit does not exceed the maximum operating temperature of the separator, e.g., the baghouse Without exotic filter bag materials, the baghouse operating temperature should not exceed about 400.degree. F. The baghouse, therefore, efficiently separates the dust and the volatilized hydrocarbons one from the other without danger of hydrocarbon contaminants condensing on the filter bags.
To the extent the dust separated in the baghouse contains hydrocarbon contaminants, those contaminants are subsequently eliminated. For example, such dust may be returned for mixing with the heated hydrocarbon contaminant-free soil exiting the second unit. The elevated temperature of that soil volatilizes any residual hydrocarbons for later elimination. The off-gases containing the residual volatilized hydrocarbons may then be conveyed to a thermal oxidizer for complete incineration and destruction. Other methods may be used to destroy these residual volatilized hydrocarbons, for example, by carbon adsorption, chemical reaction, condensation or the like. Alternatively, the dust may be remediated by other methods and later combined or not with the remediated soil.
As an added efficiency, the exhaust gases from the preferred thermal oxidizer may be disposed in heat exchange relation with the incoming volatilized exhaust gases from the separator prior to those gases passing into the thermal oxidizer. Consequently, the exhaust gases of the thermal oxidizer preheat the volatilized exhaust gases from the first-stage heating unit before they enter the thermal oxidizer, thereby economizing fuel and increasing efficiency.
Accordingly, in a preferred embodiment of the present invention, there is provided apparatus for remediating material contaminated with hydrocarbons, comprising a first-stage heating unit for receiving the contaminated material, means for heating the material in the first-stage heating unit to a first predetermined temperature to volatilize a portion of the hydrocarbon contaminants in the contaminated material and a second-stage heating unit for receiving the heated material from the first-stage heating unit. Means are provided for heating the material in the second-stage heating unit to a second predetermined temperature higher than the first predetermined temperature to volatilize substantially all of the remaining hydrocarbon contaminants in the contaminated material leaving substantially contaminant-free material. Means are provided for conveying the volatilized remaining contaminants from the second-stage heating unit to the heating means of the first-stage heating unit and means including the heating means of the first-stage heating unit are used for at least partially oxidizing substantially all of the remaining volatilized contaminants received from the second-stage heating unit. There is also provided a separator and means for conveying gases from the first-stage heating unit including the portion of the contaminants volatilized in the first-stage heating unit, any remaining non-oxidized volatilized portion of the remaining contaminants from the second-stage heating unit and any air-borne particulate matter from the contaminated material to the separator, the separator separating the volatilized gases and the particulate matter one from the other into discrete exhaust streams, respectively, and means for eliminating the hydrocarbons in the volatilized exhaust gas stream from the separator.
In a further preferred embodiment of the present invention, there is provided apparatus for remediating contaminated soil including a first-stage heating unit for receiving the contaminated soil, a burner for heating the soil in the first unit to a first predetermined temperature to volatilize a portion of the contaminants in the contaminated soil, a second-stage heating unit for receiving the heated soil from the first unit and a burner for heating the soil in the second dryer to a second predetermined temperature higher than the first predetermined temperature to volatilize substantially all of the remaining contaminants in the contaminated soil leaving substantially contaminant-free soil. Means are provided for conveying the volatilized remaining contaminants from the second unit to the burner of the first unit, in combination with means including the burner of the first unit for at least partially oxidizing substantially all of the remaining volatilized contaminants received from the second unit. There is also provided a separator and means for conveying gases from the first unit including the portion of the contaminants volatilized in the first unit, any remaining non-oxidized volatilized portion of the remaining contaminants from the second unit and any dust from the contaminated soil to the separator, the separator separating the volatilized gases and the dust one from the other. The apparatus further includes means for oxidizing the volatilized gases from the separator.
In a further preferred embodiment according to the present invention, there is provided a method for remediating soil contaminated with hydrocarbons comprising the steps of disposing soil contaminated with hydrocarbons in a first-stage heating unit, heating the soil in the first unit to a first predetermined temperature to volatilize a portion of the contaminants in the contaminated soil, disposing the heated soil from the first unit in a second-stage heating unit, heating the soil in the second unit to a second predetermined temperature higher than the first predetermined temperature to volatilize substantially all of the remaining contaminants in the contaminated soil leaving substantially contaminant-free soil, conveying the volatilized remaining contaminants from the second unit to the burner of the first unit, at least partially oxidizing substantially all of the remaining volatilized contaminants received from the second unit by the burner of the first unit, conveying gases from the first unit including the portion of the contaminants volatilized in the first unit, any remaining non-oxidized volatilized portion of the remaining contaminants from the second unit and any dust from the contaminated soil to a separator, separating the volatilized gases and the dust one from the other in the separator and oxidizing the volatilized gases separated in the separator.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for remediating material contaminated with hydrocarbons.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.